1. Field of the Invention
The present invention relates to a lens system having a diffracting surface and a refracting surface, and more particularly to a lens system advantageously adapted for the image forming optical system of an optical apparatus such as an interchangeable lens, a camera, a copying apparatus, a semiconductor-device-manufacturing exposure device, a microscope, a binocular, a telescope, an optical recording/reproducing apparatus, a projector, or the like.
2. Description of Related Art
In a lens system in which a material having a high thermal expansion coefficient, such as a plastic material, is used for a lens, variations in temperature cause a change in refractive index of the material and also cause the material to expand or contract to change the shape of the lens. The optical characteristics of the lens system would be affected by the temperature variations if such changes are left uncorrected. Therefore, it is necessary to have some correcting means. To meet this requirement, it is usual to prestore in a memory or the like the amounts of changes in optical characteristics (such as focus, etc.) anticipated to be caused by temperature variations, have a temperature sensor arranged in the vicinity of the lens, and make necessary correction by reading an applicable amount of change of the optical characteristics due to the temperature variations from the memory in accordance with the output of the temperature sensor (corresponding to the amount of change of temperature or to the temperature). However, this correcting method necessitates the use of additional means such as the temperature sensor and the memory, thereby causing an increase in cost of the lens system. To solve that problem, a method of arranging the lens system itself to be capable of correcting changes of optical characteristics due to temperature variations without using such correcting means has been developed, for example, as disclosed in U.S. Pat. No. 5,260,828. According to U.S. Pat. No. 5,260,828, a diffracting optical system (diffracting surface) such as a kinoform or the like and a refracting optical system (refracting surface) such as an ordinary lens are combined with each other in such a way as to have temperature characteristics opposite to each other.
In the lens system which is formed by combining the diffracting optical system and the refracting optical system, the color dispersion characteristic of the refracting optical system is inverse to that of the diffracting optical system. Therefore, it is known that an achromatic effect (achromatism) can be attained by arranging the power of the diffracting optical system (1/xe2x80x9cfocal lengthxe2x80x9d) to be of the same sign as that of the power of the refracting optical system (1/xe2x80x9cfocal lengthxe2x80x9d).
However, according to the specification of the above-mentioned U.S. Pat. No. 5,260,828, a relation obtained between the diffracting optical system and the refracting optical system in correcting variations of optical characteristics caused by variations in temperature is as follows. 1) The power of the diffracting optical system is at least 20% in absolute values. 2) In a plastic-used lens system, the power of the diffracting optical system is either larger than the power of the refracting optical system or of opposite sign.
Further, according to the specification of the above-mentioned U.S. Pat. No. 5,260,828, the relation between the diffracting optical system and the refracting optical system to be arranged for achromatism, i.e., in attaining an achromatic effect, is as follows. The power of the diffracting optical system is less than 15% in absolute values, and the power of the diffracting optical system is smaller than the power of the refracting optical system and of the same sign.
As mentioned above, the lens system formed by combining the diffracting optical system and the refracting optical system, as disclosed in U.S. Pat. No. 5,260,828, is arranged to do nothing for achromatism while correcting the variations of optical characteristics caused by temperature variations. In that lens system, therefore, a chromatic aberration inevitably takes place, thereby lowering the performance of the lens system.
It is an object of the invention to provide a lens system having a high rate of performance and an optical apparatus using the lens system.
In accordance with an aspect of the invention, there is provided a lens system having one or a plurality of diffracting surfaces and one or a plurality of refracting surfaces, in which a power of each surface and a distance between the surfaces are set such that an optical characteristic of the lens system, such as a focal length (focal point) and/or a spherical aberration, remains substantially unvarying relative to temperature variations within a predetermined range and that an achromatic effect is substantially attained for the optical characteristic (the focal length (focal point) and/or the spherical aberration) within a predetermined region of wavelength.
Further, in accordance with another aspect of the invention, there is provided a lens system having one or a plurality of diffracting surfaces and one or a plurality of refracting surfaces, in which a power of each surface and a distance between the surfaces are set such that an optical characteristic of the lens system, such as a focal length (focal point) and/or an amount of spherical aberration, remains the same at two different temperatures and that the optical characteristic (the focal length (focal point) and/or the amount of spherical aberration) remains the same at two different wavelengths.
Further, in accordance with a further aspect of the invention, there is provided an optical apparatus using one of the above-stated lens systems as an image forming optical system, in which the optical apparatus is an interchangeable lens, a camera, a copying apparatus, a semiconductor-device-manufacturing exposure device, a microscope, a binocular, a telescope, an optical recording/reproducing apparatus, a projector or the like.
The above and other objects and features of the invention will become apparent from the following detailed description of embodiments thereof taken in connection with the accompanying drawings.